The invention relates to a system for attaching fasteners to a workpiece. In particular the invention pertains to a system for friction or spin welding thermoplastic fasteners to a thermoplastic workpiece.
Various techniques have been used to fuse thermoplastic fasteners and workpieces together. The techniques generally involve frictionally heating the interface between the fastener and the workpiece by applying contact pressure while at the same time rotating the members relative to one another. When the interface of the two members has achieved molten state the rotation is stopped while the contact pressure is maintained for a time sufficient to allow the interface to solidiify. The length of time required to achieve a molten interface depends upon a variety of factors including fastener material, workpiece material, the size and configuration of the interface, the amount of contact pressure, the speed of rotation, the thermal dissipation characteristics of the connection, the dynamic friction characteristics of the interface, and other indeterminate factors. After the molten state at the interface is achieved, it is important to continue to apply contact pressure until the interface returns to the solid state. The time required in this step varies with the materials involved.
Typical problems encountered in friction welding include insufficient amounts of interface fusion, overheating the interface to the point were combustion takes place, and malformation of the interface due to continued rotation of the fastener during the transition from molten to the solid state.
The invention comprises a system for friction welding a thermoplastic fastener to a thermoplastic workpiece which allows an installer to apply continuous contact pressure to the fastener and the workpiece, while at the same time automatically stopping rotation of the fastener. The system also eliminates the need for the installer to judge the amount of time needed to achieve the molten state of the fastener/workpiece interface. The invention also relates to a fastener/workpiece interface configuration whereby the formation of the molten state can be controlled.
The many objects and purposes of the invention will be clear upon a reading of the following detailed description in conjunction with the accompanying drawings.